This invention relates to a coating for improving the thermal emittance of an x-ray tube anode.
It is well known that of the total energy involved in an electron beam striking an x-ray target only about one percent of the energy is converted into x-radiation while about ninety-nine percent is converted into heat. For rotating anode x-ray tubes, this thermal energy must be dissipated primarily by radiation from the target to a surrounding fluid cooled casing. Only a small amount of heat may be removed by conduction since removal of substantial heat through the rotor would increase bearing temperatures. Typically, bearing temperatures must be limited to about 500.degree. C. or the bearing alloy will soften and become inoperative.
Some diagnostic x-ray techniques now in common use apply high voltage and high electron beam current to the x-ray tube for such duration as to risk exceeding the heat storage capacity of the target and anode structure. Cine techniques, for example, are often terminated short of the desired duration because to complete an exposure sequence without allowing the target to cool would destroy it. Thus, the heat radiating capability of the target becomes a limiting factor in x-ray tube ratings. For a typical rotating anode x-ray tube, the temperature of the focal spot track of the target may be about 3100.degree. C. and the bulk temperature of the target may approach 1350.degree. C. for many diagnostic techniques. Convection cooling of a high vacuum tube is not possible so a tremendous amount of heat must be radiated through the glass envelope and, hence, to the oil circulating in the tube casing.
It is well known that thermal emittance of x-ray tube anode targets can be enhanced to some extent by roughening the target's surface outside of the focal spot track or by coating such surface with various compounds. An ideal coating would be one that has an emittance of 1.0 which is the theoretical maximum emittance of a black body. A variety of thermal emittance enhancing coatings have been used including tantalum carbide and various oxide mixtures such as oxides of aluminum, calcium and titanium. The coating materials are usually sprayed onto the refractory metal target body and fired at a high temperature in a vacuum or, in other words, at very low pressure to effect adhesion with the surface of the target. Some of these target coating materials have reasonably high emittance when they are applied but after they are fired at temperatures necessary to effect adhesion they undergo a substantial drop in emittance. It is not unusual for a material that has an intrinsic emittance of as high as 0.85 to drop down to 0.70 after processing.
Major disadvantages of target coating materials that are known to be in use up to this time are that their thermal emittance is too far below the theoretical limit of 1.0 for a black body and the coatings consist of particles which can flake off of the target when the x-ray tube is in use. These particles become positively charged during tube operation and are attracted to the electrically negative cathode. The particles cause high electric intensity fields on the cathode which reduces the ability of the tube to hold off the 150 peak kilovolts between anode and cathode which are required for tube operation.